The present invention relates to a device for position determination in sensorless motors.
In electrical machines such as electric motors, for example brushless direct current motors, which have no additional sensors for position determination of the rotor, the counter voltage UEMF of the motor winding by the electric motor force (EMF) is evaluated. The actual position of the rotor of the brushless direct current motor can be determined via the zero crossing of the alternating voltage UEMF. In brushless direct current motors with several phases, the induced counter voltages of several phases are evaluated for position determination of the rotor.
Brushless direct current motors as a rule are provided with several phases, in which the corresponding induced counter voltages can be utilized for position determination of the actual rotary position of the rotor of the electric motor. For this purpose a demultiplexer is utilized, with which the phase which is to be selected with respect to its induced counter voltage, is selected. By means of a comparator, its corresponding zero crossing is subsequently detected.
In order to match the corresponding induced countervoltage of the selected phase to the input direct phase region of the comparator, a voltage divider is utilized. Each used voltage divider has the same division ratio. In addition to the use of voltage dividers with identical division ratios, it has been found that it is possible to replace the demultiplexers with bipolar transistors in the series branch. By means of a corresponding series pass transistor, the corresponding selective phase is connected with the comparator. Since at the series pass transistors of each phase a no more negligible voltage drops, for compensation at the reference branch at the bipolar transistors provided there also a voltage drop is produced. The disadvantage of this solution is the condition that the component element tolerances and temperature influences can drive the produced voltage drop so far from one another that the phase displacement which results from it can have influence on the power and the efficiency of the brushless direct current motors.
Moreover, the solution with the use of series pass transistors in the series branches has the disadvantage that the series pass transistors provided there have parasitic capacities. In the case of a cycled control of the end stage transistors, also the corresponding blocked transistor, through this capacities voltage peaks are coupled from the phase to the comparator input. Thereby the interference voltage difference at the comparator is significantly reduced, and at low rotary speeds of the brushless direct current motor the induced countervoltage goes down into the interference spectrum and no evaluation of the input voltage signals at the comparator is possible.
Instead of bipolar transistors which can be incorporated both in the reference branch and also in each phase, also MOS field effect transistors can be utilized. With the use of the field effect transistors, for the purpose of selecting the corresponding phase to be evaluated, the voltage drop through the field effect transistor can be negligible. This however is connected with the disadvantage that the field effect transistors when compared with the effect of bipolar transistors have substantially greater parasitic capacities. The greater the parasitic capacities, the lower the interference voltage difference at the input size of the comparator. Thereby with the use of the field effect transistors in the evaluating circuit, no rotary speeds of the brushes direct current motors are no longer detected in the occurring interference spectrum, and thereby the problems which are contributed to the use of bipolar transistors in this connection are amplified.
A further solution for determination of the actual position of the rotor for a sensorless, brushless direct current motors is to implement in the phases to be selected an OR-logic with diodes. Transistors which are connected through diodes blend all not selected phases, so that one phase can be pieced at the input side of the comparator. Thereby the skipping of voltage peaks of one phase via parasitic capacities is minimized, as can occur with the use of bipolar transistors or also field effect transistors in the remaining variants. The disadvantage of the use of diodes is the condition that the voltage drops in the corresponding longitudinal paths to be evaluated can be higher than the respective voltage drop or the spaced voltage drops via component tolerances and temperature than with the use of bipolar transistors in the evaluating circuit.